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Viljuška (fork) skakača

Apr 25, 2011, 5:01 AM 0

The Double Attack.

The Knight Fork. 


(took from  Ward Farnsworth's PREDATOR AT THE CHESSBOARD)

We begin our study of tactics with double attacks, or forks: moves that attack two enemy targets at once.

And we begin our study of double attacks with knight forks. In the skeletal diagram to the left, White’s knight has forked Black’s king and rook; in other words, it attacks them at the same time.

Why start with the knight? Because it is an especially vicious and common forking tool. First, it can threaten a wide range of targets. The knight is roughly comparable in value to a bishop, and so is less valuable than a rook or queen; thus a knight not only can attack any unprotected (or “loose”) enemy pieces but also can be exchanged favorably for enemy queens and rooks regardless of whether they have protection. Second, the knight’s unique, non-straight pattern of movement creates two advantages: it allows a knight to attack other pieces without fear of being captured by them; and it enables a knight to make jumps and deliver threats that are surprising to the eye and so are easy to overlook.

To spot possible knight forks you will want to become habitually aware of the relationships between your knights and your opponent’s pieces (and between his knights and your pieces), especially as the knight progresses up the board. Every rank a knight moves forward tends to bring it closer to forking targets, especially the king; notice that once your knight reaches its fourth rank, it can attack your opponent’s back rank, and often his king, in one move (thus in the diagram to the left, White’s knight might have been on e4 a move earlier—seemingly pretty far from Black's king). Hence the strategic importance of planting knights on central and advanced squares, and the tactical importance of constantly looking for forks your knight might be able to deliver once it is properly developed.


The difficulty in fashioning a fork, of course, is that no matter where your knight sits you rarely will find a fork lying one move away against a decent player. Leaving two pieces to be forked by a knight on the next move is a blunder almost as bad as leaving a piece hanging outright. Forks have to be manufactured; the challenge is to see when one lies a few steps away. Fortunately knight forks a few steps away come in a finite number of types that you can learn to search for systematically and, with practice, recognize quickly. Such situations can be sorted into two general types.

First, sometimes two of your opponent’s pieces sit on squares that can be forked with one move of your knight, but there is some obstacle to your taking advantage of this; most commonly, the square your knight needs to reach—call it the “forking square”—is defended by your opponent (the diagram to the left shows such a case, again in skeletal form; White would like to play the fork Nf6+, but he can't; the f6 square is defended by a pawn). We will refer to these as cases where you have a potential fork—a move that amounts to a fork on its face, but that needs to be perfected by overcoming some defensive measure that your opponent has in place. In a moment we will catalogue those defensive measures and how to deal with them.

Second, sometimes you will not have even a potential fork because your opponent’s pieces are not arranged for it; there are no two enemy pieces that your knight can attack in one move. Thus in the diagram to the left, White cannot deliver a fork, but he could if he were able to get Black’s king to move over a square onto g8. In cases like this it sometimes is possible to draw enemy pieces onto forkable squares with some forcing moves—most often with a check or two. Later we will consider the clues that such possibilities for manipulation may exist and how they can be brought to fruition.

Seeing Potential Forks.

 Let's begin with ways of perfecting potential forks—in other words, cases where your opponent starts with two pieces that at least are on forkable squares. The first important thing is to see all such forks in the first place. It helps to start by learning to spot all of a knight’s possible moves at a glance. For this purpose you will want a clear mental picture of the ring of eight squares that are the maximum to which a well-placed knight can move. In the diagram on the left, the White circles show squares where the White knight can jump, and the Black circles show squares where the horribly positioned Black knight can jump. Now you can understand why having your knight near the edge of the board generally is bad policy: it can’t reach—and thus can’t control—many squares from there. Study these visual patterns so that seeing a knight’s moves from any position comes easily to you.

Now to the matter of spotting knight forks in particular. You may be used to certain forking patterns: your opponent’s king and rook are a square apart on his back rank, inviting you to fork them. But it takes more care never to overlook a potential fork when the board is crowded and the pieces to be forked are not lined up so neatly on the same row. Consider the opportunities here for Black’s knight on b7. By moving to c5 it can fork four White pieces (find them); by moving to d6 it can fork two pieces. Whether either of these forks "work" is another question (the squares the knights need are guarded, though Black has possible replies, etc.), but don't worry about that now. It's just an exercise in geometry: we want to see everyplace where two White pieces are in a forkable position. Seeing only the obvious forking candidates is no good, and won’t lead to tactical magic. If they are obvious your opponent can see them, too, and can avoid them. You want to see all of the possibilities every time they exist.

Notice an important feature of the knight's movements: every time a knight moves it lands on a different colored square. This can be used to make your searching more efficient. It means that two pieces can be forked by a knight only if they are on squares of the same color; it means that they only can be forked by a knight that lands on a square of the opposite color; and it therefore means that if a knight is in position to deliver a fork on its next move, the knight and its targets must all then be sitting on squares of the same color. This is a valuable idea; consider it a law of knight forks.

To state the practical implication plainly, one way to build your ability to see all the potential knight forks on the board is to look for any two pieces of your opponent’s that are on squares of the same color as the square where your knight sits. If, as in this case, your knight is on a light square, scan the board for pieces of your opponent’s also on light squares. Can any two of them be forked by your knight? This only takes a moment; you aren’t yet analyzing whether any of the forks would work, but just are reviewing the board visually for simple patterns—a color scan. Sometimes this will be a helpful way to alert yourself to forking opportunities; in other positions it will be more efficient just to look directly at your knight moves without reference to square color. Experiment.

As you do your scanning you will discover certain additional laws of knight moves that will become part of your visual vocabulary. An important example is that two pieces can't be forked if they are on the same diagonal with one square between them. Thus the Black king and queen in the diagram to the left are on squares of the same color, but there is no square from which a knight would be able to attack them both. This is a familiar pattern, and when you see it you will not need to pause to think about whether a knight fork is in the immediate offing; the sight of it will be self-explanatory, and you will move on.

Similarly, if your knight is on the same diagonal as an enemy piece and separated from it by one square, the knight is three moves away from being able to attack the piece. Thus in the diagram the White knight is three moves from being able to attack the Black king; it must move, say, to e4, then to g5, then to e6.

Another useful thing to know is that a knight may be able to attack an enemy target two different ways—but never more than two. In the diagram, for example, White's knight can attack the Black rook by moving to e4 or d5 (and only the latter move creates a fork). This is useful to remember because the first attacking idea you see with your knight may turn out not to be the best one—even against the same enemy piece.

Practice broad-mindedness when you scan for forking prospects. It is especially important not to dismiss a possible fork automatically, perhaps half-consciously, when you notice that the square your knight needs is protected by a pawn, or when you see that the fork would involve your opponent’s king on the one hand but a knight or protected pawn on the other. In the latter case you might quickly imagine that if you tried the fork the enemy would move his king and the pawn would not be worth taking, and so write off the forking prospect without taking it seriously. But that train of thought is premature; great combinations often look just that way at first. You want to separate the creative process of seeing that the geometry is there for a fork from the editing process of analyzing whether the fork can be made profitable. Much of the rest of this chapter is devoted to the editing process: how to take potential forks that look defective and turn them into tactical shots that work. But all along you also want to build the visual habit of noticing every time your knight can attack two sensitive points at once, no matter how implausible the attack looks at first. 

The Pinned Guard. 

When you see a possible knight fork, a natural first question is whether the square your knight needs is protected by any of your opponent’s pieces. If it is, your attention turns to the guard of the square and whether you can get rid of it—or whether you really need to get rid of it. Perhaps you don't; maybe the protection that the piece appears to offer is an illusion, as is the case if the guard is pinned. A piece is pinned if it can't move without exposing the king or another valuable piece to attack. Indeed, a piece that screens its own king from attack is subject to an “absolute” pin and so cannot legally move. We will study pins in detail in later chapters, but this much is enough to help you see that sometimes a square that looks well-defended really isn't.

So here is our method in this section: consider the piece that protects the square you want to occupy—we can call it the guard of the forking square—and see what other pieces may be on the same line with it and thus exposed to attack if it moves. Start with the diagram on the left. There is a knight fork waiting for Black with Nf2; the placement of White's king and queen with three squares between them on the first rank is a classic setup for a double attack. If that isn't yet obvious to you, notice that your knight is on a light square and that White's king and queen (not to mention several other pieces) are on light squares as well, which encourages a look at whether you can fork any of them. Having found Nf2 one way or another, ask: is f2 protected? It seems to be, by the White rook at f3; so study the rook more carefully. It's on the same line with its king, and with your queen. This means that if the rook moves it will expose its king to attack—which is to say that the rook can't legally move at all. So Nf2+ can be played with impunity, and it wins the queen after White moves his king.


Our modus operandi is to look for double attacks with the knight and ask whether they can be made to work.This time you're playing the White pieces. Notice first here that your knight is on a dark square; now look for Black pieces also on dark squares. You find the Black rook and king, and ask whether they can be forked. They can, with Nd5+. Now ask: Is d5 protected? Yes, by the pawn at c6. But before worrying further you examine the pawn to see if it is constrained. It is; it’s pinned to the king by the White rook at a6. So Nd5+ is safe, and it forks and wins the Black rook. This position is structurally about the same as the previous one.


White’s most advanced knight (generally the one you want to examine first) is on a light square. Again you might just look for knight moves, or you might look for forking candidates by scanning for Black pieces also on light squares, and find many—both of his knights, one of his bishops, one of his rooks, and his king. Nd6+ forks Black’s king and b7 bishop; the bishop is unprotected—is “loose”—making it a good target. The next question is whether the square you need (d6) is protected. It is, by Black’s bishop at e7. But then consider how the board would look if the bishop moved to d6 to take the knight. See that Black’s queen would then be taken by White’s bishop at g5; in other words, Black’s bishop is pinned to his queen. Nd6+ thus wins Black’s b7 bishop without fanfare.

There is another point to consider here. You want to think not just about what your tactical moves will achieve in the way of material gains, but also about how the board will look after the sequence you want to play. This point applies to all tactical operations; we will encounter it constantly. The important point here involves the work that your e4 knight is doing before it is sent off to inflict a fork. It's guarding the bishop on g5. To be more precise, at the start of the pictured position the bishop is protected twice (by White’s two knights) and attacked twice (by Black’s bishop and the queen behind it). The bishop therefore was safe: if Black captured it, White would recapture; if Black captured again, White would recapture again. But when White sends his knight off from e4 to d6, the bishop loses one of its guards. While this doesn’t matter so long as White is keeping Black busy with checks, notice the hazard that arises once White plays NxB at the end of the sequence. His bishop back on g5 now is attacked twice and defended only once. Does he lose it? No—but only because once his knight ends up on b7, it attacks Black’s queen. Now if Black plays BxB, White has NxQ. Black therefore needs to spend his next move taking his queen out of danger, and White’s fork works after all. The general lesson: be mindful of the defensive work your pieces are doing before you send them off to attack.


Black’s knight is on a dark square. So are several of White’s pieces, most usefully his king and e1 rook, which can be forked from f3. But notice as well that f3 appears to be protected by the rook on e3. So examine the rook and its freedom of movement, playing through its move and what would then be possible in your mind’s eye. If 1…Nxf3+; 2. RxN—and then Black can play RxRe1. White’s queen wouldn't then be able to recapture at e1 because Black would have a second rook still trained on the square. The point: White's rook on e3 is pinned—not to its king, but to the other rook at e1. One way or another Black gains a pawn and the exchange. (Capturing a rook in return for a bishop or knight is known generally as “winning the exchange.”)

Here is an important twist. Black’s most advanced knight is on a dark square. So are White’s queen, king, and rook, with the latter two pieces subject to a fork at c2. But c2 appears to be protected by White’s queen. The queen is not constrained by a pin—yet. But examine the fork by actually playing it in your mind’s eye, imagining the knight on c2 and not on b4. When you so imagine a move or exchange, pay attention to what lines are opened and closed by it and what consequences may follow—especially new pins and new possible checks. In this case, once the knight moves the White queen is pinned by Black’s queen. So play goes 1. …Nc2+; 2. Kf1, QxQ (without this intermediate step, all is lost; do you see why?); 3. NxQ, NxR. This time the lesson is that you do not just ask whether the troublesome piece currently is pinned; you ask, too, whether itwould be pinned if you made the forking move.


 A similar problem. White’s knight is on a dark square. So are Black’s king and queen. A fork is indicated at e6. The square appears to be 's protected by the pawn at d7, so look more closely; imagine the knight moved, and observe that the pawn then will be pinned by White’s queen. Ne6+ thus wins the queen without further ado. 


With White’s knight and Black’s king and queen all on light squares, conditions seem right for a fork on f6. Is the square protected? Yes, by the Black bishop on e7. If White tries to first capture it with his own bishop, then Black recaptures with his queen and the fork is ruined. But again the trick is to imagine the fork, mentally placing the knight on f6 and not on e4. Then you can see that once the knight moves, the Black bishop becomes pinned to Black’s queen by White’s queen—another “discovered” pin. The point repeats: don’t just ask whether moves are possible; picture moves, visualize whatever countermoves seem to make them impossible, and ask what would then be possible if the countermoves were made. 

Exchanging Away the Guard. 

 Now let’s assume an enemy piece guards the square your knight needs, and it isn't pinned. Perhaps you nevertheless can get rid of it. Sometimes the guardian of the forking square may be captured: you can take it, and the piece that recaptures yours no longer will protect against the fork.

Start with the diagram to the left. The position of Black’s king and rook make the idea for White clear enough: Nf7+. But f7 is protected by Black’s knight. Ask if it can be captured, and see that it can be―with White’s rook. After playing RxN, White loses the rook to f6xR; but he regains it with the fork Nf7+, capturing Black’s rook next move and leaving White a knight to the good.

Remember when you play a capture that your opponent may not be required to recapture. Usually that will be his choice, but in principle he also may be able to make some other capture or counterthreat of his own. Here Black can reply to White’s RxN by playing RxN himself. Doesn't this end the forking threat? It does, but at a prohibitive price; for then White has Re8#—a classic back rank mate that takes advantage of the way Black's king is stuck in the corner. At the outset of the position the Black rook on d8 is the only piece protecting against this mating threat, so it can't afford to leave its post. We will study back rank mates in detail at various points later in this project (they get a section to themselves toward the end).


Again one of White’s knights is pretty far advanced up the board on f5; any knight planted on the fourth or fifth rank is a constant forking threat. So White does a quick scan for forks and observes that the knight is on a light square along with Black’s king and queen―which can be forked with Ne7+. The needed square is protected by one piece: the bishop on d6, but White can take out the bishop with his rook now on d1. So White picks up a piece, and if Black recaptures White can follow up with the fork: 1. RxB, c7xR; 2. Ne7+.


The thought process is identical: White examines his knight’s moves, or perhaps does a color scan and notices that his knight and Black’s king and queen all are on dark squares; one way or another there is a potential fork in Nd7+. The hindrance is that the bishop at c8 protects the needed square. Can White capture the bishop? Yes, with his queen—a sacrifice worth making for the fork that follows. So: 1. QxB, RxQ; 2. Nd7+, and after White takes Black’s queen he has gained a bishop for his trouble. 


Your most advanced knight is on a light square, as are Black’s king and queen; there is a potential fork at e7. Ask if the square is safe, and see that it is guarded by the bishop at d6. Now look for pieces you can use to attack the bishop and notice the queen at d1―but also the knight at c4. It is important to notice both. The question is not “do you have a piece attacking X?” It’s “how many of your pieces—plural—attack X?” You don't want to sacrifice your queen when a knight will do, especially as it would make the sequence a wash. Correct is 1. NxB; c7xN; 2. Ne7+. 


The pattern repeats. White can fork three Black pieces with Ne6+. The only difficulty is the pawn at f7 that guards the needed square. There are various things one can do about such problems. The most obvious is simply to capture the pawn if you can, so here it goes 1. Rxf7, RxR, and now the pawn has been replaced by a piece that can't protect the e6 square. True, White sacrificed a rook to the cause; but now Ne6+ wins the queen. And then after Black recaptures RxN, White picks up a pawn that has been left loose by the sequence: Qxg6. White ends up trading a knight and a rook for a queen and two pawns.

You might imagine that the g6 pawn could be protected by Black's king, which (on this theory) would have escaped the knight fork by moving to f6. But if Black does move his king there, White mates in three moves. It starts with Nc3-d5+. Black has no good replies; if he plays BxNd5, for example, White has Rf1+. This forces Black to play KxNe6. Now White replies e4xBd5#.


When you capture the f7 pawn at the beginning, you should not assume that your opponent necessarily has to recapture the way you would like. He might prefer to let the pawn go rather than play into your hands; it depends on the quality of his alternatives. Here Black has the option of replying to Rxf7+ with Kg8, which loses the pawn but also takes the king out of forking range. What happens next? Imagine the board with White’s rook on f7 and Black’s king on g8, and you should see that White then has an easy capture of a piece with RxN: the rook has protection from the knight on g5, and so cannot be recaptured by Black’s king.


The usual color scan reveals a potential knight fork to be had at d6, but the square is protected. How many times? Twice—by the bishop at f8 and the knight at f7; be sure to account for all the guards, not just the first you notice. Fortunately White has bishops attacking each of the two bothersome pieces, but there still is a complication: when White’s bishops take Black’s bishop and knight, Black will use his king to recapture, and the king is supposed to be one of the pieces in the fork. Will its recaptures ruin the forking opportunity? Not necessarily; so long as the king ends up on f7 it still can be forked. But this means that it is important to perform the exchange on f7 last so that the king ends its travels there. Thus 1. BxB, KxB; 2. BxN, KxB; and now Nd6 forks king and rook and so wins the latter.
Naturally Black might prefer to bow out of this sequence earlier, giving up a piece rather than stepping into the fork. That's often how tactics work, as we will see many times (but won't always point out): the victim can escape final execution of the fork or other idea, but only by making a sacrifice. In that case—which is normal—the tactic still must be counted a success.
The point to take away from this example, apart from the importance of accounting for multiple defenders, is that the order of operations in a tactical sequence can matter a great deal. Here the tactic doesn't work if White takes Black’s knight first and his bishop second. When you consider a sequence that involves more than one exchange, ask whether changes in the order of the moves would make a difference to the outcome.
Black’s queen is about to take yours for free. Your initial impulse might be to retreat your queen or play QxQ. Maybe one of those moves is right, but don't play either until you have asked whether they can be fit into an offensive plan. White’s most advanced knight is on a dark square; so are Black’s king and one of his rooks, suggesting a fork at f7. Before you do anything you ask whether the needed square is protected, and how many times. Again, twice—by Black's bishop and queen. Unless you can eliminate both defenders of f7, no fork. Each of the defenders is attacked once: the Black bishop by your rook at d1, and the Black queen by your queen. The natural thought is to try QxQ, which you were considering anyway for defensive purposes, and then to notice that Black’s recapture, BxQ, leaves the forking square f7 loose for your knight. Capturing one of the two guardians in the right order thus takes them both out of commission. And it gets better still: once Black's bishop on d5 is out of the way, White's rook on d1 attacks the Black rook on d8. So when White plays the fork, as he now can, he can use his knight to take the d8 rook without fear of recapture by the Black rook on e8.
By moving to e2 Black’s knight would fork White’s king and queen. But the needed square is under protection—twice, from White’s knight and rook. Each of the guardians is attacked only by Black’s queen. So imagine taking out one of them, and then imagine taking out the other. If QxN, then White replies f2xQ (or BxQ) and the White rook at e4 still guards the needed square. But if Black begins with QxR, then White replies NxQ and now e2 is available for Black’s knight. Once more, an exchange of one of the two guardians effectively gets rid of both of them, as guardian #2 has to recapture the piece that captured guardian #1.
Distracting the Guard. 
In the positions we just considered, the guard of the forking square always was under attack by one of your pieces, making possible an exchange that freed up the square for your knight. But what if none of your pieces are trained on the enemy pieces doing the guarding? Then capturing the guard won't work, but there are other questions we can ask. Here's a good one: is the guard also protecting some other piece or square that you might attack, thus distracting the guard away from its defense of the forking square? This theme—distracting the guard—is one we will consider at many points in these materials; it gets a chapter to itself toward the end.
In the example on the left, the arrangement of White’s e5 knight and the Black king and queen naturally suggest a fork with Nf7. The difficulty is that f7 is guarded by Black’s rook. This time you can't capture the rook, so ask if it protects anything else you can take. It does: White plays QxN; if Black recaptures RxQ, the f7 square is left loose and available for White’s knight. The fork Nf7+ then wins back the queen with the gain of a piece.

Black has worries; he is up a piece (though White has an extra pawn), but his queen is pinned by White's rook and seems about to be lost—and his knight on f4 is hanging (in other words, it is under attack and has no protection). Look for counterplay before falling into a defensive mindset. Black’s knight and White’s king and queen are on dark squares, suggesting a possible fork at e2. Alas, e2 is protected by the knight at d4. You look for ways to take out the knight directly but find none. So now look more closely for anything else the knight protects that you might take. It guards the rook at c6, which you can capture with your queen—extinguishing the pin as well. So 1…QxR; 2. NxQ, Ne2+ and the fork is made. Black wins the exchange, having traded queens and swapped his knight for a rook.
Once you see the relationship between the rook on c6 and the potential fork far away on e2 (they are connected by the knight on d4, which guards against both of them and thus is overworked), you can choose between exploiting the situation at either end. In other words, you can (a) play the sequence just described, or (b) start with Ne2+, allowing White to play NxN, and then playing QxR since the White rook’s guard (the knight) has abandoned its defensive duties. Which sequence is better? Either way you end up ahead the exchange, but the first sequence also has the advantage of getting the queens off the board—which magnifies the significance of Black’s material edge.
That last idea is worth another minute of explanation if it's not already familiar. In simplest terms, the point is this: if you have one piece and your opponent has none, this lets you dominate the game in a way that an edge of five-to-four doesn't. So if you win a piece from your opponent and have five pieces against his four, your usual goal is to exchange away the rest of the pieces on the board, making captures when you can; finally you are left with the only attacker (at which point your opponent probably resigns, if not sooner). Naturally it follows that if you can start whittling down the number of pieces on the board while you're gaining your edge in the first place, you want to do so—as Black does here by starting with QxR.
Black's knight is on a dark square. White’s king and bishop also are on dark squares, creating a potential fork at f3; but f3 is protected by the queen at d1. Can the queen be captured? No. Is the queen protecting any other piece that can be attacked? Yes, the bishop at d4—and the bishop already is one of the pieces in the fork. So if 1…QxB, 2. QxQ, you not only have a clean fork at f3; you have a better one: Nf3+ wins the queen.

(work in progress...)

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